Stiffness and old age pretty much go together, unfortunately. Weakening muscles and stiffening joints can make it a bit more difficult for some of these old guys to get around. Still, as you’ve noted, that doesn’t deter them from getting out and doing the best that they can – it’s one of the things we love about horses, I think!
Probably the single best thing that you can do is allow your horse as much freedom to move as possible. A large corral is better than a stall – pasture is better than a corral. Moving bodies tend not to get as stiff. Of course, in a pasture setting you’re going to want to watch to make sure that he’s not getting pushed away from the feed – younger horses won’t make any concessions to the age of an older horse.
Unless you can identify a specific problem, in my experience, regular doses of non-steroidal anti-inflammatory drugs, such as phenylbutazone, can be helpful in managing these old guys. My experience with supplements, such as glucosamine and/or chondroitin sulfate, is that these products are not beneficial, and that experience is supported by an increasing amount of scientific evidence.
With good care, good food, and plenty of exercise, there’s every reason to believe that your old friend can have several more good years ahead of him!